During drilling operations, drilling fluid (e.g., drilling mud) may be pumped into a wellbore. When flowing upwards in an annulus defined between the drill string and the wellbore, the drilling mud may remove drill cuttings, reduce friction, etc., which may facilitate the drilling process. Also, depending on pressure distribution between the wellbore and the formation, the drilling mud may be loaded with formation fluids such as water, oil, and gas produced by some formations.
The drilling mud may be delivered into the wellbore through the drill string. The drill string may be rotatable, so as to rotate the drill bit, for at least a portion of the drilling operations. The drilling mud may also be used to power a mud motor within the drill string, which may be employed to provide rotation of the distal portion of the drill string. In many drilling systems, the delivery conduit for the drilling mud may be coupled to an interior of the drill string through a top drive.
During the drilling process, some connections at the top of the drill string may be broken, to add or remove drill string tubulars or pipes. For example, when drilling a new well, one or more tubulars may be added or “tripped in” when the top drive reaches the rig floor. To accomplish this, a connection between the drill string and the top drive may be broken to allow an additional tubular to be tripped in. Conversely, when a tubular is removed or “tripped out,” the opposite process is performed. For example, as each tubular is removed from the drill string, a connection at a distal end of the tubular may be broken to allow the removal of the tubular from the drill string.
When the connection between two tubulars or between the top drive and a tubular is broken during tripping in or tripping out operations, the flow of the drilling mud generally ceases. As the flow if the drilling mud ceases, however, the total pressure of the wellbore may be lowered and formation fluids may enter the wellbore. The infiltration of the formation fluids (such as gas, or liquid hydrocarbon) into the wellbore may create hazards (e.g., risk of fire or explosion at the surface) and may also affect wellbore stability. Further, the drill cuttings may settle in the annulus between the drill string and the wellbore, thereby increasing the risk of stuck-pipe. Additionally, a filter cake at the bore wall may allow the infiltration of some formations, which may reduce productivity along the reservoir and create a risk for wellbore instability. In addition, the ceasing of the drilling mud through the drill string may increase the volume of gas in the well-bore as the wellbore pressure may temporarily be lowered.